Method of estimating the signal-to-noise ratio in a telecommunications receiver and an application of the method to controlling a transmitter

ABSTRACT

A method for estimating a signal-to-noise ratio, in particular digital, received by a radio communication receiver. The method includes estimating separately the signal and the noise and filtering ( 36, 44 ) separately the signal (E b ) and the noise (N 0 ) before carrying out the division ( 40 ) of the signal from the noise. The noise filtering is for example of the statistical type, whereas the signal filtering is of the low-pass filtering type.

This application is the national phase of international applicationPCT/FR00/01943 filed Jul. 6, 2000 which claims priority to Frenchapplication 99 08 842 filed Jul. 8, 1999.

BACKGROUND OF THE INVENTION

The invention relates to a method of estimating the signal-to-noiseratio of a signal received by a radiocommunications receiver. It alsorelates to a receiver for implementing the method and to an applicationof the method to controlling the power of a transmitter.

A telecommunications system generally transmits a large number ofdifferent calls simultaneously.

By way of example, the telecommunications system considered herein isone in which terminals communicate with a control or connection station,in particular via retransmission means on a satellite. Calls betweenterminals are effected via the control station. The control stationtherefore communicates simultaneously with a set of terminals.

In the above telecommunications system, the number of calls that can betransmitted simultaneously depends on the retransmission power availableon the satellite. To maximize the capacity of the system, in other wordsto maximize the number of calls that can be transmitted simultaneously,it is necessary to minimize the power allocated to each transmitter,because the retransmission power is necessarily limited. However, thisconstraint is difficult to reconcile with the requirement to optimizecall quality, which requires sufficient transmission power. As a generalrule, the calls are digital calls and transmission quality is assessedagainst a maximum permitted error rate. The permitted error rate isguaranteed if the received signal-to-noise ratio is above apre-determined threshold.

Thus the power of a transmitter is generally determined from thesignal-to-noise ratio measured at the associated receiver and thesignal-to-noise ratio is generally measured continuously, in particularin a satellite transmission system, because propagation conditions canvary, in particular because of variations in meteorological conditions.For example, rain causes strong attenuation of the received signalcompared to transmission in fine weather. Propagation conditions canalso be degraded by scintillation due to multiple signal paths causingadditive and subtractive combination of signals. Propagation conditionscan also be degraded because of masking when an antenna is tracking amobile source (here the satellite) and obstacles block the path of thetransmitted signal.

The measured signal-to-noise ratio of a received signal is itselfgenerally subject to estimation noise and the measurements are usuallysmoothed, for example by low-pass filtering, to reduce the estimationnoise.

The accuracy of the measured signal-to-noise ratio determines thecapacity of the telecommunications system. If the measurement isaccurate, each transmitter is allocated just the necessary power, whichtherefore maximizes the communications resources, whereas an inaccuratemeasurement leads to the allocation of too much power to eachtransmitter, which is not favorable to maximizing communicationscapacity.

SUMMARY OF THE INVENTION

The invention improves the accuracy of the estimated signal-to-noiseratio and thus supplies a set point signal to the associated transmitterwhich minimizes the power it transmits.

To this end, the invention estimates the signal and the noise separatelyand the signal and the noise are filtered separately before dividingsignal by noise. It has been found that filtering each componentseparately prior to division reduces the estimation noise.

In one embodiment, the signal and the noise are filtered differently,preferably in ways that are respectively suited to the signal and to thenoise. The signal and the noise are variables of different kinds, inparticular because of their different physical origins, and so a form ofprocessing suited to one of the variables is not necessarily suited tothe other one, for example, because their amplitudes and frequency bandsare usually very different.

Also, when the traffic is sporadic, the power of the signal can beestimated only when data signals are present, although noise can bemeasured continuously.

A low-pass filter is preferably used to filter the wanted signal priorto division, on the one hand to achieve a significant reduction in thesignal estimation noise and on the other hand to achieve a sufficientlyshort control loop response time. To this end, either a finite impulseresponse filter is used, for example an averaging filter, or an infiniteimpulse response filter is used, for example a first order filter. Afirst order infinite impulse response filter is preferable in the caseof sporadic traffic because it gives more weight to more recent datathan to less recent data.

Statistical smoothing that allows for the random nature of the noise ispreferably used to filter or smooth the noise estimate. To this end, thestatistical distribution of the noise power measurements is observedover a particular period chosen to be long enough to collect a large(statistically representative) number of measurements but such that thenoise retains a static behavior during that period. A noise level abovethe average value is then chosen to constitute a limit value beyondwhich the probability of the estimated noise power exceeding that limitduring the observation period is below a low threshold ε.

In other words, to estimate the noise, instead of calculating an averagevalue, a histogram of noise levels is considered and the spread of thenoise levels is determined.

In the simplest case, the highest noise level over a suitably longobservation period is chosen, for example a period of the order of onesecond.

The noise level can also be estimated as a function of known parametersof the noise. For example, if it is known that the noise is Gaussiannoise, the average μ and the variance σ² of the distribution arecalculated and the smoothed value is μ+nσ, where σ is a standarddeviation and n is an integer such that the probability of the noisepower not exceeding the value μ+nσ is less than the low threshold ε.

More generally, the average and the variance, i.e. the moments of thedistribution, are used to estimate the noise power.

Statistical smoothing of the estimate is particularly beneficial in theevent of jamming.

It is also possible to use a low-pass finite or infinite impulseresponse noise filter, for example in the presence of thermal noise.

The present invention applies primarily to estimating thesignal-to-noise ratio of a wanted signal, i.e. of a data signal.

The present invention provides a method of estimating thesignal-to-noise ratio of a wanted signal, in particular a digitalsignal, received by a radiocommunications receiver. The method ischaracterized in that, to minimize the estimation noise of thesignal-to-noise ratio, the signal and the noise are estimated separatelyand the signal and the noise are filtered separately before division ofthe signal by the noise.

In an embodiment the filtering of the wanted signal is different fromthe filtering of the noise signal.

In an embodiment, to filter the noise signal, the statisticaldistribution of the noise power measurements is observed for aparticular period during which a statistically representative number ofmeasurement samples is collected and which is sufficiently short for thenoise to remain practically stationary.

In another embodiment the noise level used has a value such that theprobability that the noise level exceeds that value is less than apredetermined threshold during the observation period.

In an embodiment the noise value used is the maximum value over theparticular period.

In an embodiment the moments of the distribution are determined.

In an embodiment the average and the variance of the distribution aredetermined and the noise value used is μ+nσ, where σ is a standarddeviation and n is a number determined according to the predeterminedthreshold.

In an embodiment a finite or infinite impulse response low-pass filteris used to filter the noise signal.

In an embodiment a finite impulse response filter is used to filter thewanted signal.

In an embodiment the finite impulse response filter is an averagingfilter.

In an embodiment the transmitter delivers a reference signal with aregular period at a particular level and the signal-to-noise ratio isestimated from that reference signal.

In an embodiment an infinite impulse response filter is used to filterthe estimate of the wanted signal.

In an embodiment a first order auto-regressive filter is used, forexample, as expressed by the equation:{circumflex over (x)}_(i)=(1−a){tilde over (x)}_(i)+a{circumflex over(x)} _(i-1)where {tilde over (x)}_(i) represents the instantaneous estimate of thewanted signal at time i, {circumflex over (x)}_(i) represents thesmoothed estimate of the wanted signal at time i and a is an integrationcoefficient.

In an embodiment packets or cells are received sporadically and eachpacket or cell received is filtered.

The invention also provides an application of the method according tothe invention to estimating the signal-to-noise ratio in atelecommunications receiver sending data for controlling the power of acorresponding transmitter.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent onreading the following description of embodiments of the invention, whichdescription is given with reference to the accompanying drawings, inwhich:

FIG. 1 is a diagram of a transmitter and a receiver using the methodaccording to the invention,

FIG. 2 is a diagram of a telecommunications system to which the methodaccording to the invention is applied, and

FIG. 3 is a diagram explaining some aspects of the filtering used in thereceiver, shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a transmitter 10 and a receiver 12. The power P_(e) of thetransmitter 10 is determined by a set point signal δP_(e) supplied bythe receiver 12.

In the example, the transmitter and the receiver are parts of atelecommunications system in which calls are transmitted vianon-geostationary satellites 14 (FIG. 2) in low or medium Earth orbit,at an altitude of the order of 1 450 km in the example. The Earth isdivided into areas 16 each of which is 700 km in diameter, for example,and each area 16 includes a control or connection station 18, which iscentrally located therein, for example, and a plurality of terminals201, 202, etc. The connection station 18 is connected to one or moreother networks 22, for example terrestrial networks.

A call between two terminals 20 ₁ and 20 ₂ is effected via the satellite14 and the station 18. To be more precise, when the terminal 20 ₁ iscommunicating with the terminal 20 ₂, the signal transmitted by theterminal 20 ₁ is transmitted to the station 18 via the satellite 14 andthe station 18 forwards the signal to the terminal 20 ₂, also via thesatellite 14. By “satellite” is meant the retransmission means on boardthe satellite, of course.

Likewise, a call between a terminal 20 ₁ and a subscriber of the network22 is effected via the station 18. In other words, when a subscriber ofthe network 22 calls the subscriber 20 ₁, the signal is transmitted tothe station 18 which transmits it to the terminal 20 via the satellite14.

Each terminal is simultaneously a transmitter and a receiver and theconnection station 18 also transmits and receives simultaneously. Thus,in FIG. 1, the transmitter 10 is either in a terminal or in theconnection station 18 and the receiver 12 is likewise in the station 18or in a terminal 20 _(i).

A signal transmitted by the transmitter 10 propagates in space, whichconstitutes a channel 28 (FIG. 10) which attenuates the signal andintroduces noise.

In the conventional way, the receiver 12 includes a receiver unit 30, aunit 32 for estimating the power E_(b) of the signal and a unit 34 forestimating the power N₀ of the noise.

In the invention, the signal estimator unit 32 is followed by a signalfilter unit 36 downstream of a divider 40. In other words, the output ofthe unit 36 is connected to the numerator input 42 of the divider 40.

The noise power N₀ estimator unit 34 is followed by a filter 44downstream of the divider 40 whose output is connected to thedenominator input 46 of the divider 40.

The divider 40 supplies an estimate of the signal-to-noise ratio to adecision unit 50 which has an input 52 to which a reference signaly_(ref) is applied. The signal supplied by the divider 40 and thereference signal applied to the input 52 are compared to generate a setpoint δP_(e) for adjusting the power of the transmitter 10.

As an alternative to this (not shown), the decision unit is in thetransmitter and the receiver transmits the signal-to-noise ratio (theoutput from the divider 40) to a control input of the transmitter.

Consider first the situation in which the transmitter 10 is in theconnection station 18 and the receiver 12 is in a terminal 20 _(i). Inthis situation, measuring the signal-to-noise ratio is facilitated bythe transmission of a periodic reference signal from the station 18 tothe terminals 20 _(i). This is a synchronization signal of particularlevel and known period. Accordingly, in this case, the receiver 12 canuse the synchronization signal to measure the signal-to-noise ratio,instead of using the wanted signals, which are by nature sporadic.

In this case, the filter 36 for the wanted signal can be a simpleaveraging circuit performing the following operation:

${\hat{x}}_{i} = {\frac{1}{L}{\sum\limits_{j = 0}^{L - 1}{\overset{\sim}{x}}_{i - j}}}$where {tilde over (x)}_(i) represents the instantaneous estimate ofE_(b) at time i{circumflex over (x)}_(i) represents the smoothedestimate of E_(b) at time i and L is the integration length.

In this example, the filter 44 samples the noise signal N₀ with a periodof 1.5 ms over a time period of a few seconds and takes the maximumvalue observed during that time period.

As an alternative to this, over a particular time period T, chosen to besufficiently long to collect a sufficient number of measured values butsufficiently short to guarantee stationary noise behavior, theparameters associated with the distribution (histogram) of the noisesamples are calculated to deduce therefrom a noise level μ_(N0)+Δ_(N0)such that the probability that the instantaneous noise value exceedsthat level is less than ε, in other words:P(∀iε([O,T], Ñ ₀(i)>μ_(N0)+Δ_(N0))<ε

In the above equation, Ñ₀(i) represents the value of a noise sample ofthe distribution at time t_(i), T the observation period and μ_(N0) theaverage value of the noise signal.

The above equation is represented by the FIG. 3 diagram, in which theinstantaneous noise levels Ñ₀ are plotted on the abscissa axis and theprobability p(Ñ₀) of appearance of those levels on the ordinate axis.

The value adopted μ_(N0)+Δ_(N0) can be calculated using moments of thedistribution, in particular the average μ and the variance σ². In thislatter case, the smoothed value is μ+nσ, for example, where σ is astandard deviation and n is an integer chosen according to the value ofε adopted.

Then consider the situation in which the transmitter 10 is in a terminaland the receiver is in the connection station 18. In this case, theterminal does not send any periodic reference signal to the connectionstation, only sporadic data signals in the form of cells or packets, andthe signal power E_(b) is estimated in the receiver for each packet orcell. The noise can be estimated with a regular period, as in theprevious situation.

Accordingly, in this case, the filtering 44 of the noise is effected inthe same manner as under the previous hypothesis. On the other hand, itis preferable to allow for the sporadic nature of the transmission insmoothing or filtering the signal (36). A first order auto-regressivefilter is used to perform the following operation, for example:{circumflex over (x)}_(i)=(1−a){tilde over (x)}_(i)+a{tilde over (x)}_(i) ⁻¹

-   -   where {tilde over (x)}_(i) represents the instantaneous estimate        of E_(b) at time i, {circumflex over (x)}_(i) represents the        smoothed estimate of E_(b) at time i and a is an integration        coefficient.

A filter of the above kind is better suited to the sporadic nature thanan average because, as shown by the preceding equation, it gives moreweight to more recent data than to less recent data.

The method according to the invention provides an estimate of thesignal-to-noise ratio of the received signal enabling a set point to beapplied to the transmitter. It is therefore possible to minimize thepower transmitted whilst conforming to a bit error rate that does notexceed a prescribed threshold.

The above statistical processing of the noise is particularly beneficialand efficient in a situation in which the telecommunications systemshown in FIG. 2 has two adjoining areas 16 using the same carrierfrequency. In this case there is a risk of jamming in neighboring ornon-neighboring parts of the two areas and therefore of unpredictablenoise in those parts.

1. A method of estimating a signal-to-noise ratio of a digital signal received by a radiocommunications receiver, the method comprising: estimating separately a wanted signal and a noise signal of the digital signal; filtering separately the wanted signal and the noise signal; and determining the signal-to-noise ratio by dividing the wanted signal which has been filtered by the noise signal which has been filtered, wherein the filtering of the noise signal comprises determining a noise value which is used to determine the signal-to-noise ratio based on a statistical distribution of noise power measurement samples for a predetermined period during which a statistically representative number of the noise power measurement samples is collected and wherein the predetermined period is sufficiently short for the noise signal to remain practically stationary, wherein a first order auto-regressive infinite impulse response filter is used to filter the wanted signal as expressed by the equation: {circumflex over (x)}_(i)=(1−a){tilde over (x)}_(i) +a{tilde over (x)} _(i) _(i–1) where {tilde over (x)}_(i) represents an instantaneous estimate of the wanted signal at time i, {circumflex over (x)}_(i) represents a smoothed estimate of the wanted signal at time i and a is an integration coefficient.
 2. A method according to claim 1, wherein the filtering of the wanted signal is different from the filtering of the noise signal.
 3. A method according to claim 1, wherein the noise value is determined such that a probability that an instantaneous noise level exceeds the noise value is less than a predetermined threshold during the predetermined period.
 4. A method according to claim 1, wherein the noise value used to determine the signal-to-noise ratio is a maximum value of the noise power measurement samples over the predetermined period.
 5. A method according to claim 1, wherein moments of the statistical distribution are determined.
 6. A method according to claim 5, wherein an average μ and a variance σ² of the statistical distribution are determined in that the noise value used is μ+nσ, where σ is a standard deviation and n is a number determined according to a predetermined threshold.
 7. A method according to claim 1, wherein a finite or infinite impulse response low-pass filter is used to filter the noise signal.
 8. A method according to claim 1, wherein a transmitter provides a reference signal with a regular period at a particular level and the reference signal is utilized as the wanted signal to estimate the signal-to-noise ratio.
 9. A method according to claim 1, wherein packets or cells of the digital signal are received sporadically and each packet or cell received is filtered.
 10. A method according to claim 1, further comprising the signal-to-noise ratio controlling a transmit power of a corresponding transmitter based on. 